1. Field of the Invention
The invention relates to a beam positioning device for an ophthalmological instrument.
2. Description of the Prior Art
So-called laser slit lamps are known for treating retinal damage. In laser slit lamps, a treatment laser beam is coupled in between the two observation beam paths. To precisely position the laser beam on the retina, it is usual to couple in the laser beam via a deflecting element, e.g., a mirror or a prism. A micro manipulator is used to adjust the deflecting element in its angular position relative to the optical axis. Such an apparatus is described, for example, in U.S. Pat. No. 5,002,386 and U.S. Pat. No. 5,067,951. Coupling the deflections of the deflecting element and a deflecting prism with a cemented-on focusing element allows defined positioning of the laser beam. This arrangement is an expensive mechanism for ensuring a defined positioning of the laser beam on the retina.
A further possibility for positioning the laser beam on the retina is known from U.S. Pat. No. 4,776,336, which is commonly owned with the present invention. First, the treatment laser beam is coupled into the slit projection beam path via a first deflecting element. Then the coaxial laser beam path and slit projection beam path are coaxially united with the beam path of a further target laser via a second deflecting element. Positioning the point where the treatment laser is incident on the retina is brought about by defined displacement of an optical focusing element in a horizontal plane. This optical focusing element is arranged between the first and second deflecting elements. When the optical focusing element is displaced, the slit to be imaged and the point of incidence of the laser beam on the retina are simultaneously displaced in the eye. This is undesirable in various kinds of investigations. Furthermore, an arrangement of this kind is not suitable for retrofitting a conventional slit lamp with a corresponding laser attachment. Another combination of an ophthalmological instrument and a laser photocoagulator is described in U.S. Pat. No. 3,703,176. In that patent, the laser beam is positioned on the retina by laterally displacing a lens in the laser beam path. After passing through this lens the laser beam passes through a small hole in a deflecting mirror and is then coaxially aligned with the slit lamp illuminating light, which is deflected by the deflecting mirror in the same direction as the laser beam. The laser beam and the slit lamp illumating light are focused by one common lens, which is arranged between the deflecting mirror with the hole and a prism, which deflects both beams to the patient's eye.
This arrangement has several disadvantages. First, the positioning of the laser beam on the retina is limited by the dimension of the hole in the deflecting mirror. If the laser beam is laterally displaced by a greater amount, only a part of the entire laser beam diameter is available for photocoagulation on the retina. Second, if the hole in the deflecting mirror is enlarged to avoid this problem, the slit lamp illuminating light decreases. Thus, this arrangement does not position the laser beam on the retina in a satisfactory manner.